C# cannot call overloaded non-generic method from generic method

I have some legacy code with a method foo which has 700+ overloads:

[DllImport("3rdparty.dll")]
protected static extern void foo(int len, ref structA obj);
[DllImport("3rdparty.dll")]
protected static extern void foo(int len, ref structB obj);
[DllImport("3rdparty.dll")]
protected static extern void foo(int len, ref structC obj);
//and 700 similar overloads for foo...

I'd like to expose these overloaded methods through a single method using generics:

public void callFoo<T>(int len)
    where T : new()  //ensure an empty constructor so it can be activated
{
   T obj = Activator.CreateInstance<T>(); //foo expects obj to be empty, and fills it with data
   foo(len, ref obj);

   //...do stuff with obj...
}

Unfortunately this returns the errors: "The best overloaded method match for 'foo(int, ref StructA)' has some invalid arguments" and "cannot convert from 'ref T' to 'ref StructA'".

Is there an elegant way to achieve this?